Post-depositional evolution of mudstones l: early days and parental influences

Abstract

Ancient mudstones almost invariably record significant modification by very early diagenetic processes in the form of distinctive authigenic mineral assemblages. In the past, these have mostly been described from concretions—striking segregations of sulphides and carbonates self-organized into sub-spherical bodies. Thanks to relatively recent advances in electron microscopy (particularly back-scattered electron imagery) it is now a fairly simple matter to identify and characterize these same assemblages where they more commonly occur—dispersed through the fine-grained matrix of mudstones. Earliest diagenesis is almost exclusively restricted to microbially-mediated redox reactions. Within recent unconsolidated sediments, these reactions take place in vertical sequences—the 'biogeochemical zones' of Claypool and Kaplan. Causal links can now be identified between early diagenetic mineral assemblages and microbial process zonation patterns. The patterns themselves vary systematically with depositional setting. One very important aspect of depositional setting is rate of sediment accumulation. This rate control is more influential than previously suggested. It determines which diagenetic minerals will form and their amount as a proportion of the final sediment. This dependency permits early diagenesis and sea-level change to be linked through predictable lateral variation of biogeochemical zonation with sediment transport path (or bypass). It also provides clues as to the development of cementstones and horizons with discrete concretions—both of which reflect fluctuations in sediment accumulation rate. Finally, these relationships suggest why some mudstones are more likely than others to cause cementation in sandstones as a consequence of compactional de-watering and solute export.